1. Field of the Invention
The present invention pertains generally to the field of reinforced concrete construction and in particular is directed to certain improvements in three-piece couplings for end-to-end splicing of concrete reinforcement bars.
2. State of the Prior Art
A reinforcement bar or, in trade parlance, a rebar, is a long cylindrical steel rod with surface deformations or ribs, the purpose of which is to impede turning or axial displacement of the rod when embedded in concrete. One typical rib configuration includes one or two continuous longitudinal ribs traversed by a plurality of spaced apart annular ribs. Various rib configurations and designs are in use, including oblique annular and helical ribs. A summary of rebar usage may be found at pages A1 through A5 in the appendix to the "Manual of Standard Practice" published by the Concrete Reinforcing Steel Institute (Jan. 1980).
Rebars have been in widespread use for many years as reinforcing elements in poured or cast concrete structures. In construction of large structures it is often necessary to splice together two or more such rebars end-to-end. This often occurs in large concrete form-work which is carried out in steps or stages. The reinforcing bars used at each stage must be joined to rebars for subsequent stages in order to achieve a monolithic concrete structure.
Many rebar splicing devices and couplers have been devised for this purpose. In so-called three piece couplers machine threads are cut or rolled in the ends of the rebars and an internally threaded sleeve joins two bars in an end-to-end splice. This general type of coupler has long been in use. In its most basic form, crude screw threads are cut into the irregular, ribbed surface at each end of a rebar, and two such ends are joined by a threaded cylindrical sleeve. The resulting joint tends to be loose and of low quality because of the rough-cut rebar threading. A better joint is obtained by removing the ribbing to make a smooth cylindrical surface before cutting the threads cut.
Variations of this basic scheme have been adopted in efforts to improve the quality of the splice. One approach is to create a conical taper at each end of the rebar which is then threaded. The threaded ends screw into a sleeve which has equally tapered internal threading, to provide a higher grade joint.
Still another approach has been to enlarge the rebar ends by upset forging or other means, before threading is cut or rolled into the enlarged cylindrical ends. The upset forging of rebar ends was disclosed by this applicant in U.S. Pat. No. 4,619,096 issued Oct. 28, 1986, which however relates to two-piece rebar splicing, in which one rebar end is hot forged to define an integral female threaded receptacle which mates to a male upset threaded rebar end to effect a splice joint. Two-piece couplings can be advantageous in certain applications because a separate coupler sleeve, i.e. a third (piece, is eliminated. In this applicant's prior patent, it is recognized that the upset ends prevent weakening of the rebar's tensile strength after the threads are cut into the enlarged ends. This is because the rebar ends are upset or enlarged in diameter before the threads are cut. This initial thickening of the rebar ends counteracts the effect of the subsequent cutting and prevents the rebars effective cross-section from being reduced by the screw threading.
The '906 patent disclosure is, however, limited to two-piece couplings and does not address the special advantages of upset threaded rebar ends when used in conjunction with a separate coupler sleeve in three-piece couplings.
A considerable shortcoming in conventional three-piece rebar couplings is the necessity to rotate at least one of the rebars about its longitudinal axis in order to make the splice joint. In the typical situation, one rebar may already be in place, embedded in concrete, with one end protruding. The coupler sleeve is then threaded onto the exposed end of the fixed rebar, and the rebar to be joined is then threaded into the free end of the coupler sleeve to make the splice. There are occasions however, when it is inconvenient or impossible to proceed in this manner. For example, it may be impossible to rotate a twenty foot long rebar bent to a right angle at a mid-point in a confined area.
This shortcoming results from the fact that, in conventional three-piece couplers, the coupler sleeve can only be threaded a limited distance onto the rebar, which distance is the length of the threaded portion of the rebar end. This length in turn is no more than can be accepted by the coupler sleeve, as it is undesirable for smooth machine threading to remain exposed outside the sleeve in the completed splice because this results in a weak rebar section of lesser net cross-section than the ribbed rebar body. Movement of the coupler sleeve beyond the threaded end portion is blocked by the surface deformations or ribbing of the rebar which rise above the thread edges and consequently beyond the inside diameter of the coupler sleeve.
Fatigue considerations and cyclic loading presently are not a factor in the construction codes applicable to the design of concrete reinforcement bars. This is changing however and, in anticipation of stricter design codes, the U.S. Government has tested currently used rebar splices under cyclic loading and fatigue conditions. It was found that all rebar splices now on the market fall short of the performance of an unspliced reinforcement bar. It was further found that an unspliced reinforcement bar has approximately a 50% chance of meeting the new cyclic loading standards being considered. Under the old standards for e.g. highway projects such as bridges which are subject to cyclic loading by heavy vehicles passing over these structures, a rebar was acceptable if it survives 2 million cycle of 10-12 thousand psi loading. It is anticipated that the new standards will require 5 million cycles at 30,000 psi.
The ability to achieve a rebar splice through rotation of the coupler sleeve exclusively depends on the ability of the coupler sleeve to move onto the ribbed area of the rebar, unimpeded by the raised rib deformations on the rebar surface.
In the past this has been achieved by "over threading" the rebar: a first machine thread on an upset end portion of the rebar continuing the thread cut over the adjacent ribbed section of the rebar.
Empirical testing has revealed however, that even a small cut or nick in the ribbing of the rebar produces a "cherpe" effect, by which stress force acting along the rebar is focused or concentrated by relatively minute changes in the geometry of the rebar. Even a shallow thread cut in the ribs has been found to create a plane of weakening in the rebar, and under protracted cyclic loading of the rebar as may occur for example, on a concrete bridge subject to heavy loads moving across it, creates a metal fatigue condition at the site of the surface cut which in turn eventually leads to structural failure of the rebar. Consequently, the expedient of extending the machine thread onto the rib deformations in order to allow the coupler sleeve to move onto the ribbed rebar area, sometimes referred to as double threading, is undesirable if a rebar splice is to approximate the performance of a continuous rebar.
For these same reasons, the three-piece coupler system described herein is preferable to a two-piece coupler system such as described in this applicant's prior Pat. No. 4,619,096 in applications where reliability under metal fatigue conditions is of concern. The elements in a three-piece coupling are straight cylindrical rods or sleeve with no abrupt changes in geometry, in contrast to the abrupt transition between the nominal rebar area and the enlarged integral end socket in the two-piece system.